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From: Thomas Koenig <tkoenig@netcologne.de>
Newsgroups: comp.arch
Subject: Re: Computer architects leaving Intel...
Date: Mon, 16 Sep 2024 06:50:45 -0000 (UTC)
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Tim Rentsch <tr.17687@z991.linuxsc.com> schrieb:
> Michael S <already5chosen@yahoo.com> writes:
>
>> On Sun, 15 Sep 2024 20:13:44 +0200
>> David Brown <david.brown@hesbynett.no> wrote:
>>
>>> struct Bar {
>>>    char x[8];
>>>    int  y;
>>> } bar;
>>>
>>>
>>> int foo(int i) {
>>>      bar.y = 1234;
>>>      bar.x[i] = 42;
>>>      return bar.y;
>>> }
>>>
>>> It generates:
>>>
>>> foo:
>>>          movslq  %edi,%rdi
>>>          movl    $1234, %eax
>>>          movl    $1234, bar+8(%rip)
>>>          movb    $42, bar(%rdi)
>>>          ret
>>>
>>> That is, y is /not/ reloaded after bar.x[i] is set.
>>
>> No other compiler on godbolt is doing it, except possibly gcc clones.
>> Not even clang, who's former leader wrote "Nasal Manifest".
>
> Test runs on two different Ubuntu machines (gcc 7.4.0 and gcc 8.4.0)
> both show bar.y not being overwritten (optimization levels -01 or -O2)
> when foo() is called.

Same for current gcc trunk (bleeding edge development version).